1. Field of the Invention
The present invention relates to a surface mount quartz crystal oscillator in which a quartz crystal unit and an oscillation circuit using the crystal unit are arranged as a single unit to constitute the surface mount crystal oscillator, and more particularly to a surface mount crystal oscillator that prevents changes in oscillation frequency after mounting on a wiring board.
2. Description of the Related Art
Due to their small size and light weight, surface mount crystal oscillators are widely employed as reference sources for frequency and time in portable electronic apparatuses. In a typical surface mount crystal oscillator, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 2003-87056 (JP, P2003-87056A), an IC (integrated circuit) chip that constitutes an oscillation circuit is accommodated on a mounting substrate having a depression, and this mounting substrate is then bonded to the bottom surface of the crystal unit.
FIG. 1A is a partial cutaway front view of a surface mount crystal oscillator of the prior art, and FIG. 1B is a plan view showing mounting substrate 2 used in this crystal oscillator. In FIG. 1A, the mounting substrate is shown as a sectional view taken along the A—A line in FIG. 1B.
A surface mount crystal oscillator is generally composed of crystal unit 1 and mounting substrate 2.
Crystal unit 1 is a construction in which a quartz crystal blank (not shown) is accommodated in a receptacle, which is a substantially rectangular parallelepiped of low height, the crystal blank being hermetically sealed by capping with metal cover 3. Connection terminals used for connecting crystal unit 1 with mounting substrate 2 are provided at the four corners of the bottom surface of the crystal unit. The connection terminals are composed of, for example, a pair of crystal terminals for connecting electrically to the crystal blank which are arranged at both ends of one diagonal line on the bottom surface of crystal unit 1 and a pair of ground terminals for connecting electrically to cover 3 which are arranged at both ends of the other diagonal line.
Mounting substrate 2 is composed of laminated ceramics in which the depression is formed by stacking a bottom wall and a frame wall, IC chip 4 being accommodated in this depression. IC chip 4 integrates an oscillation circuit that uses crystal unit 1 and a temperature compensation circuit for implementing temperature compensation of the oscillation frequency. Connection terminals 5 for connecting with the connection terminals on the crystal unit side are formed at the four corners of the upper surface of the frame wall that surrounds the opening of the depression of mounting substrate 2. Mounting terminals 11 that are used when mounting this surface mount crystal oscillator on a wiring board are provided at the four corners of the closed face, that is, the bottom surface, of mounting substrate 2. Mounting terminals 11 include power supply terminals, output terminals, ground terminals and the like. Write terminals 12 are provided on the side surface of mounting substrate 2, these write terminals 12 being used for writing temperature compensation data to the temperature compensation circuit in IC chip 4 or for the adjustment of the oscillation frequency. All of these connection terminals 5, mounting terminals 11, and write terminals 12 are electrically connected to IC chip 4 by conductive paths (not shown) provided on mounting substrate 2.
IC chip 4 is secured by means of ultrasonic thermal compression using bumps 6 to the inside bottom surface of the depression of mounting substrate 2. Here, the surface of IC chip 4 on which circuit elements are formed confronts the inside bottom surface of the depression. Resin is then injected as what is referred to as “underfill” 7 between the inside bottom surface of the depression and the surface of IC chip 4 on which circuit elements are formed for the purpose of protecting the circuit elements formed on IC chip 4, and the resin is then cured.
The side of mounting substrate 2 having the opening in which IC chip 4 has been mounted and underfill 7 has been injected is next bonded by solder 8 to the bottom surface of crystal unit 1 such that the connection terminals on the crystal unit side and connection terminals 5 on the mounting substrate side are connected together, following which this crystal oscillator is completed by writing temperature compensation data from write terminals 12 to IC chip 4 and then adjusting the oscillation frequency, or more specifically, the nominal oscillation frequency. After completion, the surface mount crystal oscillator is mounted on a wiring board of the apparatus which is to use this surface mount crystal oscillator.
However, various apparatuses which are provided with surface mount crystal oscillators are also provided with electronic devices and electronic components in addition to the surface mount crystal oscillator, and these electronic devices and electronic components are also mounted on the wiring board. Among these electronic devices are included, for example, chip scale package (CSP) ICs. When CSP ICs are mounted on a wiring board, a protective resin for covering and protecting the CSP ICs is later applied to the wiring board. In such cases, when a surface mount crystal oscillator is adjacent to a CSP IC on the wiring board, the protective resin will also be applied to the surface mount crystal oscillator. In the above-described surface mount crystal oscillator, a gap d measuring approximately 0.05 mm exists between crystal unit 1 and mounting substrate 2, this gap corresponding to the thickness of the layer of, for example, solder 8 that was used for bonding crystal unit 1 and mounting substrate 2, and the protective resin will penetrate this gap between crystal unit 1 and mounting substrate 2. This intrusion of protective resin produces a stray capacitance between connection terminals which are provided on the bottom surface of crystal unit 1 and between these terminals and the ground potential, and this stray capacitance is added to the load capacitance from the perspective of the crystal unit, with the resulting problem of a drop in the oscillation frequency of the crystal oscillator.
Even when no CSP ICs are present on the wiring board, surface mount crystal oscillators are sometimes intentionally covered with adhesive after mounting on the wiring board for the purpose of increasing the bonding strength between the surface mount crystal oscillator and the wiring board, and in such cases, the same problem occurs in which the oscillation frequency similarly drops.